Fiber-optic sensors have been making ground of late in all areas of metrology. The reason for this is the low interference susceptibility of such sensors, especially with regard to electric and magnetic interference fields. Since it is now also possible to obtain the required optical components (optical fibers, lasers, photodiodes, etc.) with high quality at competitive prices, fiber-optic sensors have also become interesting from an economic point of view. Preference is given to piezoelectric sensor elements for the measurement of electric field strengths and potential differences. Such piezoelectric sensor elements employ the inverse piezo effect, in order to produce a change in length of an optical fiber which is proportional to the electric field strength. This change in length is then measured in an interferometric fashion.
In the publication "Optical-fiber copolymer-film electric-field-sensor", M. D. Mermelstein, Appl. Optics 22, 1006 (1983), an optical fiber is embedded in a body of PVDF (polyvinylidene fluoride). The whole is brought into an electric field, and a change in length of the optical fiber, which is proportional to the electric field strength, is measured with a Mach-Zehnder interferometer. With a given geometric arrangement, i.e. with a given field distribution, it is also possible, e.g., to measure a potential difference, since, in this case, the potential difference, e.g. between two plates of a capacitor, is directly connected to an ideal electric field strength at a given location in space.
In the publication "Fiber-optic Fabry-Perot interferometer and its Sensor Applications", T. Yoshino et al., IEEE J. of Quant. Elektr. QE-18, 1624 (1983), a voltage is measured in the manner that a fiber-optic Fabry-Perot interferometer is wound around a disc-shaped piezoelectric ceramic (PZT), a voltage is applied to the PZT, and the transmission (respectively reflection) of the fiber-optic Fabry-Perot interferometer is evaluated.
Finally, a piezoelectric sensor element is disclosed in the commonly owned Swiss patent application CH 4322/87 which corresponds to pending U.S. patent application Ser. No. 07/267,299 filed Nov. 4, 1988. Using such a sensor, it is possible to measure a specific, but freely predeterminable directional component of a local electric field. With this sensor, too, it is possible to determine a potential difference for a given field distribution.
It can be said in summary that it is possible to determine a potential difference with the known piezoelectric sensor elements for a given field distribution (electrode, conductors, insulators) through the measurement of an electric field strength. However, the fundamental disadvantage of such a measurment method lies in that it is suitable only for a fixed arrangement which does not change. In other words, the measurement arrangement must be newly calibrated for each new field distribution.